Polymer-layered silicate nanocomposites (PLSNs) have generated significant interest in academia and industry since they possess a wide range of enhanced properties. The excellent properties of such PLSNs primarily result from the use of highly anisotropic nanoscale reinforcements having particle sizes as small as 1 to 1,000 nanometers (nm) as opposed to microscale reinforcements having particle sizes as small as 1 to 1,000 micrometers (μm). The reinforcements in PLSNs can be inorganic filler particles such as layered silicates. One example of a layered silicate that has been used in PLSNs is sodium-montmorillonite (Na-Mmt), which has a crystal lattice containing two-dimensional layers in which a central octahedral sheet of alumina or magnesia is fused with two external silica tetrahedra at the tip. The thickness of the sheet is around 1 nm. These layers organize into stacks with a regular Van der Waals gap called the “interlayer” that can have a spacing of about 0.96 nm. Isomorphous substitution of the Al3+ by Mg2+ or Si4+ by Al3+ within the layered silicate can generate a net negative charge on the layer, which can be counterbalanced by alkali or alkaline earth metal cations in the galleries, i.e., the boundary region between the layers. the interlamellar cations are hydrated with one or two shells of water that surround them. Thus, the chemical formula of montmorillonite is [Mx (Al4—xMgx) Si8O20 (OH)4], where “M” is a monovalent cation, which resides in the intergallery spaces and “x” is the degree of isomorphous substitution.
Various methods have been used to fabricate PLSNs, including in situ-polymerization, solution intercalation, and melt intercalation, which is considered to be the most versatile and environmentally benign method for commercially producing PLSNs. This method involves mixing a polymer and a silicate using relatively high shear forces and heating the mixture above the glass transition or melting temperature of the polymer. The silicates used in this method are usually organically modified at the surface to improve their interactions with hydrophobic polymers, which are mostly non-polar. Organic modification can be achieved by exchanging the interlamellar alkali/alkaline ions with long chain alkyl amines or quaternary ammonium ions.